Hydrotreating of petroleum feedstocks and various boiling fractions thereof has become increasingly important because of more stringent product quality requirements. For example, governmental regulations concerning allowed limits of sulfur in petroleum products, such as diesel fuel, become more limiting each year. Furthermore, the petroleum industry foresees the time when it will have to turn to relatively high boiling feeds derived from such materials as coal, tar sands, oil-shale, and heavy crudes. Feeds derived from such materials generally contain significantly more deleterious components, such as sulfur, nitrogen, oxygen, halides, and metals. Consequently, such feeds require a considerable amount of upgrading in order to reduce the content of such components, thereby making them more suitable for further processing, such as fluid catalytic cracking, catalytic reforming, etc.
Hydrotreating of hydrocarbonaceous feeds is well known in the art and usually requires treating the feed with hydrogen in the presence of a supported catalyst at hydrotreating conditions. The catalyst is typically comprised of a Group VI metal with one or more Group VIII metals as promoters on a refractory support. Hydrotreating catalysts which are particularly suitable for hydrodesulfurization or hydrodenitrogenation generally contain molybdenum or tungsten on alumina promoted with a metal such as cobalt, nickel, iron, or a combination thereof. Cobalt promoted molybdenum on alumina catalysts are most widely used for hydrodesulfurization, while nickel promoted molybdenum on alumina catalysts are the most widely used for hydrodenitrogenation.
Further, "Novel Hydrotreating Catalysts Prepared From Heteropolyanion Complexes Impregnated On Alumina", by A.M. Maitra and N.W. Cant, Applied Catalysis, 48 (1989) pp. 187-197, teaches hydrotreating catalysts prepared by impregnating alumina with solutions of heteropolyanions having the general structure [H.sub.w A.sub.x B.sub.y O.sub.z ].sup.n-, where A may be Co or Ni, and B may be Mo or W. This reference also shows that these catalysts were tested for hydrodesulfurization and hydrodenitrogenation activity and were found to have no significant advantage over standard commercial hydrotreating catalysts.
While catalysts containing molybdenum with nickel, cobalt, or both, are in extensive commercial use today, they have limitations with respect to removing heteroatoms from heavy feeds, such as heavy coker gas oils and coal derived gas oils. As the feeds become heavier, the content of condensed aromatic hydrocarbons, with and without heteroatoms, increases. These condensed aromatics can absorb strongly on the catalyst sites reducing both the rate and extent of heteroatom removal. Consequently, there exists a need in the art for improved hydrotreating catalysts having increased activity toward such heavy feeds, particularly when the heteroatom to be removed is sulfur or nitrogen.